Control of air vehicles is often achieved using discrete control elements such as flaps, slats, ailerons etc. Various arrangements have also been suggested for changing the aerodynamic profile of components, such as a wing, without employing a discrete element. These arrangements, in which the overall profile of the component is altered, are often referred to as ‘morphing skin’ designs. Morphing skins may provide many advantages such as a reduction of the complexity of the high-lift systems, decrease in maintenance expenses and the supply of a lighter system for air vehicles which leads to reductions in operating costs, e.g. fuel. In addition, more environmentally friendly air vehicles could also be achieved because of lower emissions of pollutant gases such as CO2 and NOx due to lower fuel consumption.
Several morphing skin concepts and technologies have been suggested: In a first example, the use of elastomers for a morphing skin has been suggested because their stretchable structures allows for a change in surface area, an important property required for morphing skins. However, elastomers are not suitable for carrying and transferring aerodynamic loads to the underlying structure because their tensile strength is only around 50 MPa. In another example, deployable materials such as rolling, collapsible, foldable and inflatable structures have been considered because they provide a change in surface area when they are actuated. However, like elastomers, they are also unable to cope in high load environments, such as aircraft wings. Moreover, it is difficult to achieve a smooth surface during deployment.
Some multi-stable composites have been suggested for use in morphing skins because they are able to change their shape due to stiffness controlling. However, aerodynamic loads tend to be greater than the forces required to change these multi-stable composites from one stable arrangement to the other, and the multi-stable composites have been found to therefore be unsuitable for morphing skins.
Shape memory materials such as shape memory alloys (SMA), shape memory polymers (SMP) and elastic memory composites (EMC) have also been suggested for use in morphing skins because their stiffness can be changed, which results in a shape change. Shape memory alloys (SMAs) have been found to provide a low actuation rate. Shape memory polymers (SMPs) have been found to have a low stiffness, durability, and toughness which means that it is difficult to use them as structural materials. Elastic memory composites (EMCs) also suffer from the same problems as SMPs. Reinforcing SMPs has been suggested to improve their stiffness but this has resulted in a reduction of maximum strain.